The invention relates to a piston for a cylinder, in particular a brake piston for a brake cylinder, made of a plastic containing a filler and a method of manufacture thereof.
Pistons made of plastic are known here and there from the state of the art. Such pistons, in particular brake pistons, often must be able to withstand high pressures and high temperatures during their use. In order to achieve a high strength and heat resistance, plastics are used for the pistons which contain up to 80% fillers. Such a filler can, for example, be rock flour. However, the disadvantage of such a plastic piston is that it can be torn by the cylinder. The piston can then no longer rest sealingly on the cylinder wall and the hydraulic fluid provided in the cylinder exits from the cylinder.
A further disadvantage of such a plastic piston is that it can be manufactured at best with a tolerance of 0.09 millimeter. However, in order for the piston-cylinder unit to function satisfactorily, a deviation of 0.015 millimeter at a maximum may exist. In order for a plastic piston to be able to be utilized, same thus needs to be machined so that the necessary diameter is obtained. A further disadvantage can be that unforeseen chemical-physical interactions can occur between the plastic of the piston and the hydraulic fluid, which interactions can influence the efficiency of the piston.
The basic purpose of the invention is thereby to provide a piston which cannot be damaged by the cylinder wall, which can be manufactured inexpensively within the prescribed tolerances, and in which a contact between the hydraulic fluid and the plastic is prevented. A further purpose relates to the accurately fitting manufacture of the piston.
This purpose is attained according to the invention by the piston being provided with a container made out of metal, by the outer surface of the piston being at least partially covered and being brought into contact with the cylinder wall.
The piston of the invention provided with a container meets the two requirements demanded in the purpose of the invention. The container prevents, on the one hand, a direct contact between the plastic of the piston and the cylinder wall so that the piston is not damaged and, on the other hand, the container can be manufactured with such preciseness in the dimensions thereof that the piston equipped therewith meets the demanded tolerances. Furthermore a contact between the plastic of the piston and the hydraulic fluid is prevented, thus eliminating a physical-chemical interaction between the plastic and the hydraulic fluid.
The container can thereby be glued according to the invention to the outer surface of the piston. Grooves extending in direction of the piston axis are then advantageously provided in the outer surface of the piston, which grooves can receive an adhesive. As an alternative it is also possible for the container to be connected to the piston through a mere frictional and/or form lock achieved by a press fit relation therebetween.
The piston has in an advantageous embodiment a hole extending in direction of the piston axis. The hole can then have according to the invention one or several undercuts. The braking shoes of a braking system can then, for example, be secured to these undercuts. The undercuts can moreover according to the invention extend to the bottom of the hole.
However, the hole in the piston can according to the invention also be a through-hole. The through-hole can then be closed off with an insertable bottom member.
The bottom of the hole can according to the invention have one or several recesses. This saves weight, in case the recesses forming the holes are already formed during an injection molding process, and also material.
The bottom of the container can advantageously rest on the bottom of the hole.
The purpose relating to the manufacture of a piston of the invention is attained by initially forming a container-shaped semi-finished product, the outside dimensions of which are equal to or slightly less than the outside dimensions of the piston to be manufactured, by subsequently inserting the semi-finished product into a tool which has the outer contour of the piston to be manufactured, by thereafter filling plastic under high pressure and, if necessary, high temperature into the container-shaped semi-finished product, thus causing an expansion of the semi-finished product to the outside dimensions prescribed by the tool, and by finally removing the container filled with the solidified plastic from the tool.
The container-shaped semi-finished product, which in its outside dimensions is slightly smaller than the finished piston, is inserted into a tool. This tool, advantageously an injection-molding tool, has exactly the dimensions which the piston to be manufactured is supposed to have. The semi-finished product is filled under high pressure with plastic, which causes the walls of the semi-finished product to be pressed outwardly until they rest against the tool. The container receives thus exactly the desired outer contour, while the plastic of the piston is fed clearance-free into the container. Also a subsequent fastening of the plastic in the container becomes unnecessary. A quick and economical manufacture of the piston is possible with this method, thus eliminating a finish-working because of the precise manufacture.
The container has advantageously a corrosion protection property or consists of a fine steel.